The invention concerns a method for thermal conversion of methane to hydrocarbons of higher molecular weight, and the arrangement for carrying out the method. More particularly, it concerns a method for conversion or thermal cracking of methane in a reactor which has electric heating means and which enables acetylene, ethylene, benzene and a small amount of coke to be produced through dehydrogenating thermal coupling of that molecule.
All methane sources which are well known in the art may be used. A very common source of methane is natural gas. A non-exhaustive list of these sources has been provided, for example, in Applicants' European patent application EP-A-323287. In most cases the gas containing methane which is fed into the reactor contains from 1 to 90% of at least one other gas, and sometimes even more.
In European patent application EP-A-323287 Applicants have described a method for thermal conversion of methane to hydrocarbons of higher molecular weight, comprising electric heating means with heat being transferred to the gas mixture containing the methane to be converted, through the impermeable walls of ceramic sheaths which insulate said heating means from the gas mixture containing the methane. In this method the heating zone is heated through providing electricity by means of electric resistors, and the heat liberated by the Joule effect in the resistors is transmitted, chiefly by radiation, to the ceramic sheaths arranged non-contiguously around the resistors. The gas charge, which circulates substantially perpendicular to the axis of the heated sheaths, is heated essentially by convection and by radiation. In establishing this method two spaces are defined in the reactor:
firstly the reaction space or processing space, outside the sheaths protecting the resistors, in which the gas mixture containing methane circulates,
secondly the resistor space, formed by the volume between the resistors proper and the insulating sheaths, into which an inert gas is preferably introduced, i.e. a gas without any methane or any hydrocarbon liable to undergo a thermal conversion reaction or any compound liable to react violently with methane or hydrogen. The gas is also chosen so that it does not damage the resistors used or accelerate the ageing of the resistors.
One of the most serious problems in thermal conversion of methane is bound up with coke formation. When too much coke is formed there is the danger of damaging the furnace before de-coking operations are carried out and, from an economic point of view, coke formation represents a serious loss in respect of both the electricity consumed and the methane consumed in forming it. This problem, which is well known in the art, is partially resolved by including hydrogen in the gas mixture containing the methane to be converted, the hydrogen representing from 1 to 90% by volume of the total volume of gas. But coke formation, chiefly on the walls of the sheaths and other high temperature surfaces in contact with the gas mixture containing methane, is not completely eradicated despite this precaution.
This explains why the following is desirable when carrying out this method for conversion of methane in an electrically heated pyrolysis oven:
to have a relatively large quantity of hydrogen in the processing zone,
to have electric resistors which can deliver a large amount of energy per unit area per unit time at high temperature,
to have conditions which allow for good heat transfer, so that the temperature of the heating elements, and that of the sheath surfaces in contact with the mixture of gases containing methane, is not too far above the temperature required to convert the methane.
It has been specified that, in carrying out the method, it is preferable for the resistor space to be filled with a gaseous fluid such as nitrogen, carbon dioxide gas or air. The use of air can only be considered if the sheaths form a perfect seal between the processing space and the resistor space. Otherwise there would be a serious danger of forming a very high temperature gas mixture containing oxygen, methane and hydrogen, with a consequent risk of explosion. The creation of a perfectly impermeable system is extremely difficult and also necessitates using highly impermeable and thus very high quality ceramics, that is to say, ceramics of a density close to the theoretical density, without any open pores.
Such ceramics are very expensive to use, which is a great disadvantage of the method. One therefore has to accept the use of sheaths which are not completely impermeable, and of either nitrogen or carbon dioxide. If nitrogen is used with silicon carbide resistors, there is a danger of silicon nitride forming; this is more than a negligible danger in view of the skin temperature of the resistors. In theory this has no effect on the mechanical strength of the resistors, but it changes the resistivity of the heating elements and thus accelerates their ageing, all the more since their temperature is higher and they are providing more energy. In the case of carbon dioxide, even if there is not much leakage from the resistor space to the processing space, trouble will inevitably be experienced in separating the products formed during thermal conversion of the methane: separation will be complicated a) by the presence of the carbon dioxide and b) by the presence of carbon monoxide and of water, which is inevitably formed by reaction between carbon dioxide, methane, coke and hydrogen in the processing space.